A thermoplastic bag for storing food items may include a first sidewall and an opposing second sidewall joined to the first sidewall to define an interior volume. The first and second sidewalls may be made from pliable, sheet-like thermoplastic web material. The sidewalls may be formed with a pattern of ribs. When a tensioning force is applied to the sidewall, the ribs may open so that the sidewall may become generally more planar. Opening the ribs may cause the sidewalls to stretch or expand thereby adding to the overall area of the bag. The thermoplastic material may have a shape memory characteristic that may cause the ribs to reform after the tensioning force is alleviated.